Magnetically controlled microwave structures



W. A. YAG ER March 5, 1957 MAGNETICALLY CONTROLLED MICROWAVE STRUCTURESFiled May l0, 1952 2 sheets-sheet 1 Ww F M/CROWA VE CARR/ER GENE/M TORV, R REM m6 m Nm T FAA WA. WZ Vr B ....1 Ahi March 5, 1957 w. A. YAGER2,784,378

MAGNETICALLY CONTROLLED MICROWAVE STRUCTURES Filed May l0, 1352 2Sheets-Sheet 2 MAGNETICALLY CNTROLLED MICROWAVE STRUCTURES William A.Yager, New Providence, N. J., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York Application May10, 1952, Serial No. 287,152

7 Claims. (Cl. 332-51) This invention relates to the phenomenon offerromagnetic resonance absorption and more particularly to themodulation of microwave energy and the reduction of microwaveattenuation in ferromagnetic materials.

One object of this invention is to utilize the antiresonant or minimumabsorption operating range of the ferromagnetic absorptioncharacteristic to reduce microwave attenuation.

Another object of this invention is to utilize the steep slope adjacentthe minimum absorption point for modulation purposes.

A further object of the invention is to improve the etliciency ofmagnetically controlled microwave components.

in accordance with one aspect of the present invention, the objects setforth above are realized byV making certain of the walls of highfrequency resonators of ferromagnetic material, and these walls aremagnetized, at critical values of lield strength. The walls which are tobe magnetized are selectedV because of their relationship with theparticular microwave field configuration in the wave guide. In theillustrated embodiments to be described in detail hereinafter, certainresonant cavity Walls are formed of magnetic material and are magnetizedat right angles to the high frequency magnetic lield at the surface ofthe particular Wall.

In accordance with another aspect of the invention, magnetic plates arelocated in a conductively bounded passageway for electromagnetic waveenergy, and an induction device is provided for applying a magneticfield to the plates.

The nature of the present invention and various objects, features, andadvantages in addition to those noted above will appear more fully fromthe following descrip.- tion of the embodiments shown in the drawings,in which.:

Fig. l is a plot of apparent permeability against magnetic eld strengthfor a particular magnetic material;

Fig. 2 illustrates a modulation system in accordance with the invention;

Fig. 3 represents cylindrical microwave resonant charnbers of thetransmission type which may be utilized in the system of Fig. 2;

Fig. 4 depicts an alternative resonant chamber with permanent magnetbiasing;

Fig. 5 shows a rectangular transmission type resonant chamber inaccordance with the invention;

Fig. 6 is a cross section of the resonant cavity of Fig. 5 and alsoshows a magnetic yoke to the device of Fig. 5; and

Fig. 7 represents a reection type resonant cavity and a matched hybridjunction which may also be used with the system of Fig. l.

As set forth by the inventor and his associates in a number of articles,including C. Kittel, Phys. Rev. 71, pagev 270 (1947); 73, page 155(19.48); W. A. Yager and R. M. Bozorth, Phys. Rev. 72, page 80 (1947);W. A. Yager, Phys. Rev. 73, page 1247 (1948); 7S, page 316 (1949); W. A.Yager and F. R. Merritt, Phys. Rev. 75,

page 318 (1949); such curves of apparent permeability plotted againstmagnetic iield strength, as shown in Fig'. 1, are known inthe art. Inthe case of Fig. 1, the frequency involved is 24,000 megacycles, thematerial is annealed Supermalloy, and the magnetic eld strength is thestatic magnetic tield in the Supermalloy at right angles to thetangential component of the high frequency magnetic Y iield.

To summarize the results set forth in certain of the above-enumeratedarticles, it has been determined that the point of antiresonance orminimum apparent permeability will occur when U=7B where:

w=the angular frequency 'i1-magneto mechanical ratio, and B=the magneticinduction.

Analyzing each of these quantities,A in greater detail, w=21rf, where fis theA frequency of the radio frequency magnetic eld.

The magneto mechanical ratio n 'lf-2me where g is the so called g orsplitting factor, e and m are the charge and mass of an electron, and cis` the velocity of light.

The magnetic induction B=H..-I%+B. where Hap is the applied magneticfield, N is the demagnetizing factor which may be determined for a givenspecimen, as set forth at pages 320 to 326 in the Journal of AppliedPhysics, May 1942, vol. 13, and B; is the saturation induction for theparticular material employed.

In any particular case, it may thusl be determined what applied magneticfield is required for antiresonance.

A pertinent prior art device is disclosed in the patent to King2,197,123, issued April 16, 1940. The magnetic attenuator in this patentis a section of a straight transmission wave guide and is not operatedat any critical level of magnetization.

The foregoing being a summary of what is old and known in the art, theparticular devices illustrated in this application will now be discussedin greater detail.

Initially, in accordance with the invention, a substantial portion ofthe internal surface area of a resonant cavity is formed of magneticmaterial. These cavity walls are magnetized in the directionperpendicular to the direction of thev greater portion of the highfrequency magnetic iield adjacent the particular wall, at or close tothe critical magnetic field required for antiresonance. A cavityresonator is used rather than the walls of a straight transmission waveguide, because a given percentage change in permeability results in amuch` greater change in reflected or transmitted power for the formerthan the latter.

Fig, 2 represents Ia schematic diagram 0f a modulation system employingl[he invention. In this figure, a microphone 1, shunted by the highresistance 2, excites the amplifier 3, The transformer 4 matches theimpedance of the amplier to the modulation coil 6 which encircles theresonant cavity 5. The biasing coil 9, which is energized by the directcurrent power supply 7 through the variable resistance 8, also eniirclesthe resonant cavity 5. As will be described in greater detail inconnection with Figs. 3-6, the apparent permeability of certain magneticwalls of the cavity S vary with applied magnetic field. As the apparentpermeability increases or decreases, the transmission loss of the cavity5 increases or decreases correspondingly. Energy from the microwavecarrier generator 10 is coupled to the variable loss cavity 5 by thewaveguide 11. The resonant cavity 5 thus serves to modulate themicrowave energy generated by the carrier wave source 1i) in accordancewith variations in the modulating and biasing current in the coils 6 and9. "The speech modulated waves are then transmitted through the outputwave guide 12 to the antenna 13.

Fig. 3 represents one form of transmission type resonant cavity S whichmay be used with the system of Fig. 2. In this figure, the wave guide 11is the input to and the wave guide 12 is the output from the rightcircular cylindrical cavity 5. The two coupling slots` 14 and 15 arelocated olf center in the end' plates 17 andlS and serve to excite thecavity in a TMoin mode. When energized in this mode, the componentlofthehigh frequency magnetic held adjacent the cylindrical wall is parallelto the two ends walls 17 `and 13. When the cavity 5 ofFig; 3 isassociated with the circuit of Fig. 2, the coils 6 and 9 set up amagnetic liux lengthwise in the ferromagnetic cylinder 16 which makes upa large portion of the internal surface of the cavity. This longitudinalmagnetic tield in the cylinder is perpendicular to the high frequencymagnetic field in the cavity adjacent the cylindrical inner surface, andthus is properly oriented to obtain the ferromagnetic resonanceabsorption effects set forth hereinbefore. The length of the cylindricalcavity is an integral number of half wavelengths ofthe excitationfrequency in order to resonate in a TMom mode. Inasmuch as the endplates 17 and 13 are not in the magnetic circuit, they may be ofconducting non-magnetic material. t l v Referring to Figs. l and 2,thecurrentiiowing in th biasing coil 9 could, for example, be adjustedto the point where the plot of permeability vs. magnetic field strengthis very steep. The negative slope region of the curve in the -l50oersted section of the plot -is perhaps the best section of the curvebecause of the low steady biasing flux required and the steep slope ofthe curve. Thus, with a reasonably low level of modulating current inthe coil 6, a substantial modulation of the microwave carrier can beeiected by the substantial change in the loss of the cavity 5.Modulation by biasing to a point on either side of the resonanceabsorption peak would, of course, Ialso be satisfactory.

Fig. 4 represents a cross section of an alternative cavity arrangementin which permanent magnet biasing is employed in place of the separatemagnetic coil 9 shown in Fig. 2. The carrier Wave is introduced to theright circular cylindrical modulating cavity 16, 19, 2! through the waveguide 11 and is coupled to it by a slot which is off-center in the endplate 20, and thus asymmetric with respect to the center line 22 of thecavity. Similarly, the modulated microwaves are coupled to the outputwave guide 12 by means of an off-center coupling slot in the end plate19. As in the embodiment of Fig. 3, the cylindrical surface 16 is ofmagnetic material. In lthis case, however, the end plates 19 land 20 areof magnetic rather than conducting non-magnetic material to provide amagnetic path for the steady biasing ilux from the permanent magnet 21to the magnetic cylinder 16. The coil 6, which encircles the magneticcylinder 16, introduces the modulating iiux which is superposed on thesteady linx of the permanent magnet just` as the modulating flux fromthe coil 6 is superposed on the direct current liux of the coil 9 in theembodiment of Fig. 2.

Figs. and 6 show another alternative transmission type resonant cavityin accordance with the invention. Fig. 5 is an isometric view of thewave guide and resonant cavity, while Fig. 6 is a transverse crosssection of the resonant cavity and also shows the external magneticcircuit for the cavity. Referring now to Fig. 5, the wave guides 11 and12 again represent the input and output to and from the resonant cavity.Suitable transition elements 41 and 42 are employed to match the waveguides to the resonant cavity. As seen to advantage in Fig. 6,

the top 44 and bottom 44 of the resonant cavity, as well as theintermediate plates 45, are of magnetic material and are energizedtransverse of the direction of microwave propagation by the modulatingcoil 46 and the biasing coil 47 through the magnetic yoke 48 andthemagnetic side plates 43 and 43'. With the cavity energized in the TEnmmode, the transverse magnetic field in the plates 44 and 45 is seen tobe perpendicular to the greater portion of the high frequency magneticiield adjacent these plates. The side plates 43, which are not directlyinvolved in the antiresonance phenomenon, may be covered with conductingnon-magnetic plates 49 to reduce the over-all losses in the cavity.

Fig. 7 represents still another modulating resonant cavity arrangement,in this Icase a reflection type cavity. The inherent properties of amatched hybrid junction are used in connection with this embodiment. Theparallel hybrid arm 11 and the series arm 12 represent the'input andoutput from this modulation arrangement. The arm 51 of the hybridjunction is provided with the impedance matching and absorbingtermination 53 as is well known in the art. The reflection typelresonant cavity 54 is coupled to the hybrid junction by means of thewave guide 52 aligned with the arm 51. Applying the known properties ofhybrid junctions to this device of Fig. 7, the input energy in arm 11will split evenly between arms 51 and 52, with no direct energy passingfrom the parallel arm 11 to the series arm 12. In addition, half thereflected energy from the cavity 54 will be transmitted to the outputseries Iarm 12. The apparent permeability of the cavity 54 is varied bymeans of the coil 6, as explained in connection with Figs. 2-4, and isadjusted to a suitable operating point by means of a permanent magnet ora steadily energized coil. The reliected energy from the cavity is, ofcourse, a function of the `apparent permeability of the cavity, and, asthis is changed in accordance with the modulated signal, the amount ofenergy transmitted from the hybrid junction through the series arm 12will vary.

The above-described resonant cavities could, of course, also be used 'aslow loss cavity resonators merely by ernploying a proper steady biasingux so that the device lwould operate precisely at the antiresonantpoint. With suitably low values `of minimum `apparent permeability, itappears that cavities with even less loss than copper or silver would bepossible.

While particularly well adapted :for use in connection 4with modulatorsor low loss cavity resonators, it is to be understood that theabove-described arrangements are merely illustrative embodiments made inaccordance with .the invention. Numerous other arrangements may bedevised by those skilled in the art without departing from the spiritand scope of the invention.

What is claimed is: l

1. A rectangular cavity'resonator having a plurality of spaced magneticplates extending substantially across said cavity, means forestablishing a biasing magnetic lield in said plates, and a wave guidecoupled to said cavity.

2. A rectangular cavity `for electro-magnetic waves having a pluralityof spaced magnetic plates extending substantially across said cavity,means for establishing a biasing magnetic teld in said plates, and awave guide coupled to said cavity.

3. In combination, a longitudinally extending wave guiding 'passage-Way,a plurali-ty of plates ol magnetic material located in said passagewayland having the greater portion .of the surface area thereof spaced fromthe walls of said passageway, said plates being oriented parallel to thelongitudinal axis of said passageway, induction lmeans for applying `abiasing Vmagnetic field to said plates, 'and means `for introducingmicrowave energy into said passageway in a mode -in which the magneticcomponents of said microwave energy Yare perpendicular' to said biasingmagnetic eld.

4.In combination,- '.a first elongated, conductively bounded passageway,a plurality of spaced magnetic plates extending substantially from oneconducting bound- Aary of said passageway to another, said plates beingsubstantially parallel with the longitudinal axis of said passageway,means for establishing a biasing magnetic eld in said plates, and secondand third conductively bounded passageways connected to said rstpassageway at opposite ends thereof to -transmit electromagnetic waveenergy through said rst passageway.

5. In combination, Ia rst elongated, conductively bounded passageway, aplurali-ty of spaced magnetic plates located within and having theirwide surfaces spaced from the walls of said passage-way and beingoriented substantially parallel to the longitudinal axis of saidpassageway, means for establishing a biasing magnetic eld in saidplates, and second and third conductively bounded passageways connectedto said first passageway -at opposite ends thereof to transmitelectromagnetic wave energy through said rst passageway.

6. In combination, a longitudinal extending conduce tively boundedstructure for electromagnetic wave energy, a plurality of spacedmagnetic plates located within said structure and being orientedsubstantially parallel to the longitudinal axis of said struc-ture,means for establishing a biasing magnetic field in said plates, andsecond and third conductively bounded structures connected to said rststructure at opposite ends thereof to transmit elec- Itromagnetic waveenergy through said rst structure.

7. In combination, a rst elongated, conductively bounded passageway, aplurality of spaced magnetic plates exten-ding substantially from oneconducting boundary of said passageway to another, said plates beingsubstantially parallel with the longitudinal axis of said passageway,means for establishing la steady biasing magnetic deld in said plates,means for superposing a variable magnetic field to said steady biasingfield, and second and third conductively bounded passageways connectedto said first p-assageway at opposite ends thereof to transmitelectromagnetic wave energy through said rst passageway.

References Cited n the tile of this patent UNITED STATES PATENTS2,197,123 King Apr. 16, 1940 2,233,263 Linder Feb. 25, 1941 2,402,948Carlson July 2, 1946 2,483,818 Evans Oct. 4, 1949 2,510,016 Fernsler May30, 1950 2,511,610 Wheeler June 13, 1950 2,629,079 Miller et al. Feb.17, 1953 2,652,541 Cutler Sept. 15, 1953 2,671,884 Zaleski Mar. 9, 1954OTHER REFERENCES Microwave Resonance Absorption in FerromagneticSemiconductors, by Hewitt, Physical Review, May 1, 1948, vol. 73, N0. 9,pp. 1118, 1119.

